The most widely used configuration for modern aircraft control surfaces generally consists of one or several essentially planar elements located such that they form the leading edge or trailing edge of the aerodynamic lifting surfaces of the aircraft. The aircraft is controlled by means of the deflection of the control surfaces, which causes a change in the outer geometry of said aircraft resulting in aerodynamic direction and magnitude forces suitable for carrying out the control.
The change of aerodynamic shape required to control the aircraft is generally achieved by means of the rotation of said control surfaces around a hinge or rotating shaft fixed with respect to the aerodynamic lifting surface to which they belong. There are other configurations and methods to carry out the control, for example by means of the elastic deformation of the entire lifting surface, a method used by the Wright brothers in the first airplane as described in document U.S. Pat. No. 821,393; by means of a complete rotation of the lifting surface as described in document U.S. Pat. No. 6,089,503; by means of deformation of the lifting or control surface caused by changes in the properties of the material as described in documents U.S. Pat. No. 6,209,824B1, U.S. Pat. No. 5,662,294; or by means of air blasts or engine exhaust gases, such as in the case of the AV-B Harrier airplane, or the North American X-15.
The configurations of the control surfaces consisting of rotating said surfaces around a hinge have been used in aeronautics in most airplanes and are the only ones currently used for large passenger transport airplanes. The first airplanes to use this control surface configuration, as well as most lightweight airplanes today, use a cable and pulley system to transmit the pilot's control actions, either directly using the force exerted by the pilot or through a servo system mechanically amplifying the pilot's force. The cable and pulley systems move the control surfaces through a lever system converting the linear movements of the cables into rotations of the control surfaces. This actuation method for actuating control surfaces is suitable for lightweight aircraft or those of a larger size flying at relatively low speeds (much less than the speeds near the speed of sound at which commercial aircraft currently fly), due to the fact that the forces which the cables can transmit are relatively low and the aerodynamic forces acting on the control surfaces, and which must compensate the forces of the cables, linearly grow with the area of said control surface and with the square of the flight speed. The cable and pulley system has additional limitations due to the inherent flexibility of the system, formed by long cables with a small section, which can lead to aeroelastic instabilities if it is applied to large control surfaces, in addition to introducing a delay in the operation of the control surfaces and a possible lack of response from the flight controls when the plan flies at high speeds, all this due to the lengthening of the cable system introduced by the aerodynamic loads.
With the development of aeronautic technology it was necessary to develop new actuation methods for actuating the flight control surfaces particularly adapted to large airplanes flying at increasingly greater speeds, generally driven by reaction engines. The adopted solution consisted of using servo-actuators, necessary for exerting the high control forces required in order to move large control surfaces at high flight speeds and to place the mentioned actuators in a position such that they could transmit the control forces directly to the control surfaces, representative of a rudder, or to a typical aileron or elevator installation.
The configuration of the previous typical aileron or elevator installation has the evident drawback of requiring an aerodynamic fairing for the actuator, which is an unwanted source of aerodynamic drag. On the other hand, this configuration has the advantage that the leading edge of the control surface is very close to the rear stringer of the lifting surface to which it is associated (generally the wing or the stabilizers) therefore allowing the maximum sectional area of the respective torsion boxes to be used, which results in an increase in the rigidity of said boxes, particularly the torsional rigidity and furthermore, where applicable, the maximum volume of the fuel tank in the case of a wing or horizontal stabilizer.
The configuration representative of a rudder, typical for a modern commercial airplane rudder, does not require an aerodynamic fairing for the actuator, but however has the drawback of significantly reducing the available space between the torsion box of the lifting surface and the control surface. In all cases this involves an unwanted reduction of the torsional rigidity of both elements (main torsion box and aerodynamic control surface). The separation between the rear stringer of the torsion box and the leading edge of the control surface likewise requires installing relatively large and flexible aerodynamic fairings which do not contribute to the rigidity or resistance of the lifting surface, in addition to introducing large bending loads on the ribs of the torsion box at the base of the hinge fittings, all of which is undesirable.
Reducing the area of the section of the torsion box of the lifting surface, imposed by the previously described and necessary separation in order to install the actuator, usually results in a weight increase of the structure since thicker skins and stringers are required in order to restore the desired torsional rigidity for aerodynamic and aeroelastic considerations.
The problem of elastic deformation of the mentioned surfaces under aerodynamic load has to be solved in all flight control systems based on the rotation of the control surfaces. In cable and pulley systems, in which the levers to which the control cables are connected are usually at one end of the control surfaces, the aerodynamic loads cause torsional deformation on the control surface tending to take away control effectiveness. In order to restore control surface effectiveness, the surface's torsional rigidity must be increased, either by increasing the thickness of its structural skins (which adds weight and increases the inertia of the control surface, both unwanted consequences, i.e. weight for aircraft efficiency reasons and inertia for tending to reduce the speed at which dynamic aeroelastic instability or fluttering occurs), or by using a torsion bar near the leading edge of the control surface, which also adds weight but limits the increase of the moment of inertia of the surface.
In the case of systems in which the actuators are connected directly to the control surfaces, said actuators are usually located approximately in the middle of the span of the control surface in order to minimize torsional deformation, or several actuators are used in parallel, which also provides the control system with redundancy. In any case, the placement of the actuators within the aerodynamic surface requires providing accesses thereto for inspection, which complicates the design of said aerodynamic surfaces, and in the case of the rudders makes access difficult for maintenance personnel.
The object of the present invention is to solve the previously mentioned problems with regard to making control surfaces, particularly those associated to the configuration in which the actuators are directly connected to said control surfaces, since this is the configuration used for control surfaces in stabilizers in large modern commercial airplanes.